Equation of a plane and normals + Decals

In my uDev entry I have decided to make roads a separate object on top of the height-map terrain. Right now I have a road that is in the correct X and Z position but the Y position is a constant 15. What I want to do while loading is take each vertex and keep their X and Z location but using the planes normal, find the Y location so the point is on the plane. How would I do this?

Also, once I get that fixed, should I add something like .02 to the roads height to make sure it is drawn instead of the terrain, or is their a better way to do that so the road is always drawn slightly above the terrain (I don't want them to merge together far away, which is what I am afraid might happen).